The present invention relates to the preparation of structures for use in molten carbonate fuel cells. Generally, molten carbonate fuel cells are comprised of a cathode, an anode, and an electrolyte structure disposed between the cathode and the anode. The electrolyte structure is composed of carbonate electrolyte and a supporting matrix. Under fuel cell operating conditions, at temperatures usually in the range of from about 500.degree. C. to about 700.degree. C., the supporting matrix acts to retain, by capillary action, the electrically active molten carbonate between the cell electrodes. The present invention relates to both the preparation of electrode structures as well as the electrolyte structure.
Fuel cells employing molten carbonate electrolyte can accept various carbonaceous gases as fuels. For example, methanol and carbon monoxide along with hydrogen have been proposed. One source of a fuel gas is that produced in the gasification of coal which includes carbon dioxide, carbon monoxide and hydrogen. In such a cell, the following reactions occur. At the anode: EQU H.sub.2 +CO.sub.3.sup.- .fwdarw.CO.sub.2 +H.sub.2 O+2e.sup.- EQU CO+CO.sub.3.sup.- .fwdarw.2CO.sub.2 +2e
At the cathode: EQU 2e.sup.- +CO.sub.2 +1/2O.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.-
The carbon dioxide gas required at the cathode can be provided from that produced at or delivered to the anode, all as well known in the Kinoshita et al. U.S. Pat. No. 4,115,632 issued Sept. 19, 1978 for METHOD OF PREPARING ELECTROLYTE FOR USE IN FUEL CELLS, the disclosure of which is incorporated herein by reference and the Arendt et al. U.S. Pat. No. 4,216,278 issued Aug. 15, 1980 for PROCESS OF MAKING ELECTROLYTE STRUCTURE FOR MOLTEN CARBONTE FUEL CELLS, the disclosure of which is incorporated herein by reference.
The active electrolyte material is generally provided as a mixture of molten alkali metal carbonates or mixtures of alkali metal carbonates and alkaline earth carbonates at the cell operating temperature. Considerable reduction in melting temperatures can be obtained by using eutectics and other molten mixtures of the carbonates, as is well known in the art.
Molten carbonate fuel cells have been suggested as stacks of repeating elements. Each element contains an anode, a cathode with an electrolyte structure or compact separating the two. Anode structures can include porous, sintered nickel possibly alloyed with chromium or cobalt. Suitable means of current collection and an electrically conductive separator plate between the anode and the next cell in the stack are incorporated. Cathodes of similar structure are contemplated of, for instance, porous nickel oxide prepared within the cell by oxidation of sintered nickel structures. The electrolyte structure disposed between the electrodes includes the active electrolyte material of metal carbonates along with an inert matrix or substrate material. The alkali metal aluminates particularly lithium aluminate are currently of interest for use as this inert substrate material. The formation of lithium aluminate (LiAlO.sub.2) is favored relative to sodium or potassium because of its greater stability. Another inert substrate material commonly used in carbonate fuel cells is strontium titanate (SrTiO.sub.3). One of the principal problems in fabricating either electrodes or electrolyte compacts for carbonate fuel cells has been the cracking of the structure upon cooling after introduction of the carbonate into the matrix. This in part is due to the fact that the coefficient of expansion or contraction of the matrix is different than the carbonates, that is the volume change on solidification of the molten electrolyte coupled with the differential thermal expansion between the matrix material and the electrolyte material are the primary causes of distortion and cracking. Therefore, in view of the problems associated with the prior art methods of preparing carbonate containing structures for use in molten carbonate fuel cells, it is an object of the present invention to provide a process for making porous carbonate-containing structures for use in molten carbonate fuel cells which prevents the cracking and distortion of the matrix material upon cooling.
It is a further object of the present invention to provide a process for producing such carbonate containing structures than is applicable to the electrolyte compacts as well as the electrode structures. It is also an object to provide a process which is operable with inexpensive materials with ordinary operating temperatures without the need of exotic equipment.